I only began to use gmail because all my workmates are doing so. I was not so thrilled but later I found it very useful.
1- It always works even though internet connection slows (which happens often in the country where I am).
2- It is the only mail where you can attach big files. If you use yahoo, it will take some time and you are not even sure it will work.
3- It’s very easy, which is a plus for new users like me :D.
4- You have multiple sign in.
5- You have google + which is I think another version of facebook.
6- gmail messenger is the best messenger I’ve seen. You can have video/audio conference call, group chat only by using your mail account.